The problem of fluid loss to highly permeable underground formations penetrated by a well has been long recognized. These highly permeable zones are often called thief zones. In water or steam stimulation operations, for example, a serious problem is often encountered because a very small interval of the total production zone may be taking 80 percent or more of the total injected fluid. When this happens, the benefit of the injection project may be lost or greatly reduced.
An isolated high-permeability zone or fracture can be plugged at the well bore face by a shallow layer of applied cement, though such a permanent relatively irrevocable technique often is undesirable. More desirably, a communicating high-permeability zone is plugged to some considerable depth in order to prevent flood water from otherwise merely flowing around a narrow shallow plug and back into the high-permeability or thief zone. Plugging of a relatively high-permeability zone to a substantial depth converts the zone into a much lower permeability zone. Then, subsequently injected flood water or other fluid will tend to enter the formerly by-passed by now relatively more permeable hydrocarbon-bearing zones and thus mobilize increased amounts of hydrocarbons.
Various methods have been used in the past to achieve in depth plugging, such as gelable systems triggered by a following aqueous acidic solution injection for subsequent pH adjustment. However, injecting an acidic solution following the gelable solution may result in such rapid gelation that sufficient in depth plugging is not obtained in the most permeable strata where desired. In another method, water, a polymer and a cross-linking agent capable of gelling the polymer such as a sequestered polyvalent metal cation, are admixed, and, just before injection into an underground formation, an acid is added thereto to effect gelation. But, when the acid is pre-mixed with the gelable composition, the gelation reaction can be too fast, making it necessary to shear the gelled polymer in order to obtain adequate injection, which reduces effectiveness of the gel. Also, gels are limited as to structural integrity. A gel may fill up the pore space of a given zone, however, gels do not have adequate mechanical strength. They deform under load (100 to 5000 psi water pressure) and allow water to pass, just as before gel treatment.
A wide variety of polymers and copolymers have been used for permeability reduction with varying degrees of success. These polymers include among others, the materials disclosed in the patents listed as prior art. The disclosed polymers suffer from the disadvantage that they all shrink or decrease in volume on polymerization. When such materials are pumped into a zone of high permeability and set up, the adhesion of the polymer is poor, which allows fluid channeling through or around the polymer. The polymers generally will have good mechanical strength and will hold up under load. However, the shrinkage which occurs during polymerization of monomer to polymer leaves open pore space in the high permeability zone. As a result, water at 100 to 5000 psi will easily channel through the polymer. The degree of shrinking encountered varies with the polymer, e.g. ethylene polymer shrinks 66%, vinyl acetate 21% and 1-vinylpyrene 6%.
According to this invention, substantial reduction in the permeability of a highly permeable zone in an underground formation is achieved by introducing into the formation through a well bore a polymerizable material which remains constant in volume or expands on polymerization and thereafter allowing polymerization of such material to take place. If necessary, a catalyst is used to effect the polymerization reaction.